A Practical Introduction to Data Structures and Algorithm Analysis

364 Chap. 10 Indexingkey value for each block in that cylinder, called the cylinder index. When newrecords are inserted, they are placed in the correct cylinder’s overflow area (in effect,a cylinder acts as a bucket). If a cylinder’s overflow area fills completely, thena system-wide overflow area is used. Search proceeds by determining the propercylinder from the system-wide table kept in main memory. The cylinder’s blocktable is brought in from disk and consulted to determine the correct block. If therecord is found in that block, then the search is complete. Otherwise, the cylinder’soverflow area is searched. If that is full, and the record is not found, then thesystem-wide overflow is searched.After initial construction of the database, so long as no new records are insertedor deleted, access is efficient because it requires only two disk fetches. The firstdisk fetch recovers the block table for the desired cylinder. The second disk fetchrecovers the block that, under good conditions, contains the record. After manyinserts, the overflow list becomes too long, resulting in significant search time asthe cylinder overflow area fills up. Under extreme conditions, many searches mighteventually lead to the system overflow area. The “solution” to this problem is toperiodically reorganize the entire database. This means re-balancing the recordsamong the cylinders, sorting the records within each cylinder, and updating boththe system index table and the within-cylinder block table. Such reorganizationwas typical of database systems during the 1960s and would normally be doneeach night or weekly.10.3 Tree-based IndexingLinear indexing is efficient when the database is static, that is, when records areinserted and deleted rarely or never. ISAM is adequate for a limited number ofupdates, but not for frequent changes. Because it has essentially two levels ofindexing, ISAM will also break down for a truly large database where the numberof cylinders is too great for the top-level index to fit in main memory.In their most general form, database applications have the following characteristics:1. Large sets of records are frequently updated.2. Search is by one or a combination of several keys.3. Key range queries or min/max queries are used.For such databases, a better organization must be found. One approach wouldbe to use the binary search tree (BST) to store primary and secondary key indices.BSTs can store duplicate key values, they provide efficient insertion and deletion aswell as efficient search, and they can perform efficient range queries. When there